Conventional human-powered transportation systems, such as bicycles, generally include a pedal drive system wherein a user rotates a crankshaft by applying force against pedals connected to crank arms with the user's feet. Conventional bicycle drive systems typically include a forward sprocket, or chainring, attached to the crankshaft and a rear sprocket, or cog, linked to the forward sprocket by a tension chain or cable. Opposing crank arms extend from opposite sides of the forward sprocket, and pedals are pivotally attached to each crank arm end. The user then rotates the forward sprocket in one angular direction by applying revolutionary force against the pedals. As the sprocket rotates, the tension chain or cable causes the rear sprocket to turn. When the rear sprocket is linked to a wheel, forward motion is achieved.
Such conventional drive systems for human-powered applications have been in existence for hundreds of years and, while advances in braking, gearing and shifting systems are common, the basic configuration including a forward sprocket, a rear sprocket and a single tension chain extending between the two has persisted as the base technology.
Conventional human-powered transportation systems of the types described above have a number of drawbacks. One problem associated with conventional human powered transportation systems, and particularly conventional bicycles, is the finite number of available gear ratios. For example, a conventional multi-speed bicycle may have two or three front sprockets and six or seven rear sprockets. A user may shift between the different sprocket combinations by causing the chain to derail from one sprocket and engage another, but oftentimes it would be most efficient for the user to operate the bicycle using a gear ratio between two existing gears. However, the unavailability of gear ratios between those defined by the forward and rear sprockets results in lost efficiency and power where intermediate gear ratios are needed.
Another problem associated with conventional drive systems for bicycles and other types of human-powered transportation relates to the ability of a user to revolve the user's leg in one angular direction. In conventional drive systems for bicycles, a crankshaft generally includes a right crank arm and a left crank arm extending from the crankshaft on opposite sides of the bicycle. The user's right and left legs turn the crankshaft by pedaling forward. As the crankshaft turns, a front sprocket, or chainring, turns. During use, the crank arms are pedaled in only one angular direction to provide torque to the rear wheel. A user's legs must travel in a circle as each crank arm rotates about the crankshaft in the bottom bracket of the bicycle. It is well known that, by increasing the length of the crank arms, i.e. the distance between the point of force application, i.e. the pedal, and the crank shaft, or bottom bracket, a greater force may be applied to the front sprocket, or chainring. As a result, more torque can be applied to the rear wheel, as seen generally in FIGS. 12A and 12B. However, because conventional bicycle drive systems require a user's legs to move around in a complete revolution, the possible crank arm length is limited by the user's anatomy and clearance with the ground. Using the conventional design, when the crank arm length is extended beyond a threshold distance, it becomes uncomfortable or even impossible for a user to make a complete turn of the crank arms while positioned on the bicycle.
What is needed then are improvements in the conventional devices and systems for human-powered transportation, and particularly for drive systems for bicycles.